The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Homogenous charge compression ignition (HCCI) engines combust an air/fuel (A/F) mixture within cylinders to produce drive torque. HCCI engines may combust the A/F mixture in different combustion modes. For example, in an HCCI combustion mode the A/F mixture may be automatically ignited when compressed by pistons (i.e. compression ignition). Alternatively, for example, in a spark ignition (SI) combustion mode the A/F mixture may be ignited by spark plugs in the cylinders after the pistons compress the A/F mixture.
HCCI combustion mode may improve engine efficiency and/or fuel economy compared to SI combustion mode. However, HCCI combustion mode may be limited to a predetermined HCCI operating zone in order to reduce combustion noise and protect the engine from damage due to excessive pressure increases associated with HCCI. Therefore, pressure sensors may be implemented in one or more of the cylinders and may be used to monitor cylinder pressure, particularly during HCCI combustion mode.
Furthermore, HCCI combustion mode may require precise A/F ratio control to prevent increased emissions. More specifically, lower peak temperatures during combustion (compared to SI combustion mode) may lead to incomplete burning of fuel. Therefore, carbon monoxide (CO) and/or hydrocarbon (HC) pre-catalyst emissions may be higher during HCCI combustion mode than in spark ignition combustion mode. For example, the increased CO and/or HC emissions may be higher during HCCI combustion mode due to incomplete oxidation and/or trapped crevice gases, respectively.